There is increasing utilization of nonhomogeneous materials in engineered applications. Wooden lumber and wood-based composites are widely used in structural applications. Synthetic composite materials, such as fiber-resin composites also are now used in substantial amounts. However, such materials are not being utilized to their fullest potential because of difficulties in accurately determining the allowable working stresses to which structural members made of these materials should be subjected.
Much of the difficulty preventing full utilization arises because these nonhomogeneous materials do not exhibit uniform strength or elasticity properties from specimen to specimen. Wood materials by nature include numerous knots, separations, grain variations, and inherent differences in the strength of the wooden fiber matrix. Composite materials suffer significant variations in bonding between reinforcing fibers and the resin filler or other matrix or binders. Wood-based composites such as particle boards, wafer boards, plywoods, glued laminated beams and others combine some of the difficulties of both. Accordingly, it has been impossible to provide strength or elasticity grading information which allows for full use of relatively stronger members contained within a group or class of structural members having significant variations in strength, elasticity or other mechanical properties. More typically, the allowable working strength and elasticity properties have been predominantly determined by the weakest members in a class.
Also of concern is the increasingly widespread use of lumber in structural applications in which the lumber is stressed in tension, as compared to bending or compression. For example, chords of trusses, wooden I-beams, and tensile laminae of glued laminated timbers are primarily used under conditions were the primary potential failure mode is in tension. This has generated serious questions concerning the validity of assigned strength and elasticity values which have traditionally been almost exclusively based on strength testing in bending or flexure. Most of the allowable working stresses assigned to lumber have been derived from bending tests wherein the strengths are initially determined by bending the members to failure. This basic bending failure strength information is then extrapolated through theoretical analysis and assumptions to predict corresponding strength and elasticity properties in tension. Because of the relatively large factors of safety incorporated in allowable working stress values this lack of relationship to actual tensile testing has in most cases not caused disastrous results. However, the more efficient utilization of wooden members strongly suggests the need to have more accurate engineering models and information for designing members subjected to different types of stress.
The increased utilization of lumber in tension applications is also rendered more problematic because the tensile properties of lumber are more difficult to predict than are bending properties when using the lumber grading systems, visual grading systems and machine stress rated system currently used in lumber production. These current grading systems are unable to assess tensile properties with sufficient precision due to inefficiencies in the relationships between the detected flaws, proof testing techniques, or force-deflection response characteristics used in such systems and their appropriateness for predicting tensile properties. Such prior grading systems typically sort relatively high strength pieces into grades of lower strength and value because they are unable to identify the higher tensile strength members. The discrimination criteria used thus underutilizes a majority of the pieces tested.
Current lumber grading technologies also suffer from limitations associated with their inability to grade green lumber or lumber otherwise having a moisture content greater than 19%. This limitation leads to vast amounts of wasted energy because lumber which is intended for use in special applications is dried prior to grading for strength. This leads to costly drying of a substantial amount of lumber which is not of sufficient strength for such higher uses. If such relatively weak lumber could be graded out prior to drying then substantial economic benefits and energy savings could be realized.
Wooden and composite materials are not being utilized to their fullest potential and great economic losses are associated with not selectively using stronger and better members where they have the greatest value. As the value of these materials increases the need becomes more acute to properly grade nonhomogeneous structural members based upon their particular strength and elasticity capabilities.